The structure of the Milky Way and our place in it
The Milky Way is a barred spiral galaxy about 100,000 light-years across, containing hundreds of billions of stars. Its main parts are a thin disk with spiral arms, rich in gas, dust, and young stars; a central bulge of mostly older stars crossed by a bar; a supermassive black hole at the very center, called Sagittarius A*, with about 4 million times the Sun's mass; and a roughly spherical halo of old stars and globular clusters, surrounded by an even larger halo of dark matter. Our solar system sits in the disk, in a small spiral arm segment called the Orion Arm, about 26,000 light-years from the center, and orbits the center at roughly 230 kilometers per second, taking about 230 million years for one trip around. Because we view the disk from inside it, the Milky Way appears in our night sky as a glowing band stretching across the sky.
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